During filling and storage of powdery material, such as cement, paper sacks are required to meet high standards.
Firstly, the paper sacks need to hold a considerable material weight, i.e. have high tensile strength. For this purpose, Kraft paper is a suitable sack wall material. The sacks typically have two or more walls, i.e. layers of paper material, to further strengthen the sack construction. A wall layer of a sack is often referred to as a ply. Production of ply material (i.e. sack paper) is for example disclosed in WO 99/02772.
Secondly, a material such as cement is sensitive to moisture contamination during storage. Hence, cement sacks often require protection against atmospheric water vapor penetration through the sack plies. Such protection is often achieved by a moisture barrier incorporated as an intermediate layer in the sack, i.e. between two plies of the paper material. The moisture barrier is typically a plastic film (“free film”), e.g. of polyethylene (PE), that is impermeable to water. The free film may also improve resistance to grease and prevent contamination by microorganisms.
Thirdly, the paper sack should vent air during filling. In detail, the air that accompanies the powdered material shall efficiently vent from the sack as the filling machines that delivers the material run at high throughput rates. Often, the venting capability of the sack is the actual limiting for the filling rate. Efficient venting also prevents that air is trapped in the sack and causes under-weight packs, sack rupture and problems when sacks are stacked for transportation.
During the filling process, the only way for air to escape from the interior of the sack has, in many sack constructions, been through the walls of the sack. Kraft paper of high porosity is often used in the walls to facilitate air permeability. However, an increased porosity of the paper normally results in a decrease in the overall strength. In particular, the strength may be significantly reduced if holes must be made in the paper material to achieve sufficient air permeability. Furthermore, the use of a free film may reduce deaeration during filling, since most such films are impermeable to air. Therefore, the free film layers have been provided with slits or openings to facilitate deaeration.
Traditionally, construction workers have opened the cements sacks and added their contents to a mixer. However, some alternative solutions have been suggested.
GB2448486 discusses a dissolvable bag made from paper or another soluble materials, to contain building materials requiring mixing, such as cement, lime or plaster. It is stated that the dissolvable bag can be directly added to the mixer where the bag rapidly dissolves when water is added, which reduces spillage, wastage, mess and exposure to the building products. The packs are placed together in a waterproof wrapping to ensure the product stays dry whilst stored or in transit. However, no moisture barrier in the bags is discussed.
WO 2004052746 suggests spray or dip-coating the whole exterior of already filled bags with a non-permeable, waterproof coating. It is further suggested to place the bag in a mixer also containing a quantity of water, wherein resulting ingress of water into the bag causes a water-soluble inner layer of the bag to dissolve, thereby allowing the waterproof exterior of the bag to disintegrate within the mix. WO 2004052746 fails to device any materials for the inner and outer layer of the bag.
US 2011/0315272 states that a sack that dissolves in a moist environment can be obtained by using a dextrin adhesive for gluing the folded ends of the sack. Folding and gluing patterns for the ends are also discussed. A moisture barrier in the sack is not discussed.
JP5085565A suggests that a cement sack that can be added directly to a mixer is composed of a water soluble material, such as PVOH, having a thickness of 20-70 μm. FR2874598 discloses a similar solution.